This invention is concerned with the rotary method of oilwell drilling wherein hydraulic fluid, usually "mud", is pumped down the drill string and onto the bottom of the hole to clean the bit rollers and to flush the chips up the well bore. A great deal of effort has been expended on bottom hole cleaning coupled with drill bit design, in order to increase penetration rate, and mud pressure and its hydrostatic head is a controlling factor in this drilling efficiency. The weight of the mud is controlled and related to the bottom hole pressure required because of the pore pressure, represented by "formation pressure gradiant", that presents an instantaneous boundary pressure interface of mud-to-formation at and surrounding the drill bit. The formation pressure gradiant varies with depth and type of formation entered into, and it can change rapidly and unexpectedly resulting in "blowouts" or "kicks", which has required blowout prevention equipment installed at the surface as a first line of defense. With these factors in mind, the driller is normally required to proceed with control over weight and pressure to establish an "over balanced" condition by substantially equalizing or exceeding the formation pressure at the bottom of the hole, and it is generally accepted by drillers that in deep-hole drilling the bit hydraulics has a major effect upon the rate of penetration. It is also accepted that the rate of penetration can be increased greatly with "under balanced" conditions at the bit, but normally with the risk of cave-in and uncontrolled implosion of the well bore at and surrounding the lower end of the drill string. In view of the foregoing, it is a general object of this invention to provide a pressure differential drilling tool and method of drilling for increasing the rate of penetration in deep holes.
The formation pore pressure at the bottom of a well bore varies with the depth and the type of formation, and which can be calculated by using as a factor the "formation pressure gradient" of the formation to be penetrated. The average formation pressure gradient in the continental United States is 0.465 p.s.i. per foot of depth: and the maximum abnormal therefor is accepted as 1.0 p.s.i. per foot of depth. The "formation pressure gradient" can be defined as the compaction pressure, the pore pressure, which the formation exhibits and from which condition it can unexpectedly release as in the case of a sudden opening from the formation and into the well bore. With an under balanced condition the formation will become sensitive and tends to implode, for purposes of this invention.
As stated above, the formation pressure gradient is important in the drilling process because it is a factor which determines the need for pressure and hydrostatic head applied in order to keep the formation intact and thereby prevent collapse of the well bore. Conventional bit nozzles or jets are another factor, as they restrict flow and control circulating fluid pressure. Accordingly, pump pressure, jet restriction and hydrostatic head pressure of the mud are the controlling factors employed to establish "over balanced" and "under balanced" conditions as related to the pore pressure and the formation pressure gradient thereof, it being an object of this invention to provide a drilling tool for pressure differential drilling that subjects the well bore bottom interface to an under balanced condition, while subjecting the drill string above the bit to a higher or an overbalanced condition. With the present invention, the differential condition is immediate to the bit, whereby cutting action at the bit-to-bore bottom interface is under balanced for rapid penetration, and whereby the hole above the bit is at higher pressure and/or over balanced for normally accepted pore pressure-mud column equilibrium.
The pressure differential tool and method of drilling herein disclosed is characterized by the pressure separation below and above the bit, thereby establishing distinct pressure zones, it being an object of this invention to substantially isolate the bit zone from the drill pipe zone during the drilling operation, so that an underbalanced condition can prevail at the bit-to-bore bottom interface, and so that a higher pressure or an over balanced condition can prevail above the bit. Accordingly, a modulating plug is provided at and above the bit to closely fill the annulus between the bit (sub or drill collar) and the bore through which the modulating plug continuously advances as hole is made. In practice, a controllably expansible elastomer boot is employed through which the drill string rotates. However, when hydraulic drill motors are employed the drill string is fixed and need not rotate through the modulating plug, although so called mud motors can rotate through the modulating plug as it is disclosed herein.
The present invention requires the controllably expanded modulating plug to slideably engage the well bore closely behind the drill bit, it being an object of this invention to advantageously utilize the modulating plug as a stabilizer. In the normal practice of rotary drilling long lengths of drill collar are commonly used together with winged stabilizers, in order to make straight hole. With the present invention, the bore engageable modulating plug close above the drill bit inherently centers the bit in the well bore, and by stabilizing the drill string with an additional stabilizer substantially above the modulating plug the amount of drill collar can be greatly reduced, as the active portion of the drilling string is then turning on spaced centers. In practice, the lower modulating plug-stabilizer and the upper stabilizer are on sleeve members that turn on the drill pipe or tool bodies through anti-friction bearings, as will be described. Alternately, a non turning drill pipe locates a mud motor on centers (not shown).
It is an object of this invention to control the above mentioned modulating plug during lowering of the drill string and during the drilling operations, and to this end the plug is expansible only in response to the application of hydraulic pressure, and specifically through the application of mud pump pressure. A significant feature is the inclusion of a releasable check valve that bypasses fluid within the tool for rapid descent into the well bore, said valve being removeable so as to open up the tool to fluid passage. The modulating plug is hydraulically operated in response to pressure build-up at the jet pump nozzle, established when mud pump pressure is controllably applied.
A primary object of this invention is to generate a differential pressure in the separated and distinct zones below and above the bit, actually below and above the aforesaid modulating plug immediately above the bit. The drill bit and drill string apparatus is essentially mechanical-hydraulic in its operational functions, and it is the inherent presence of dynamic fluid under pressure that is employed to generate the differential pressure to establish an under balanced condition distinct from a balanced or over balanced condition. A feature of the present invention is the inclusion therein of at least one peripheral nozzle jet pump operable in the tool with control over the bore modulating plug. The jet pump per se operates without moving parts and is capable of lifting a greater volume of fluid than is required therefor through its ejector features for operation. However, it is the differential in pressure between the intake or suction tube (plenum) and discharge mixing throat (venturi) with which this invention is primarily concerned, a differential calculated to reach substantial proportions.
An object of this invention is to provide a well drilling tool with a surrounding bore modulating plug, while advantageously utilizing the tool interior to establish the jet pump features. Characteristically, the body of the tool has cross-over passages that pass drilling fluid and chips to be carried away by the upward flow thereof. A feature is the internal bypass of downwardly flowing circulating fluid from the interior of the drill string and around the jet pump and supplying a nozzle plenum of the jet pump. A feature is the sharing of said internally bypassed circulating fluid under pressure from the interior of the drill string with the modulating plug (static) to controllably expand the same to substantially close the well bore annulus surrounding the tool; and responsive to mud pump pressure that simultaneously controls the static pressure applied to expand the modulating plug and dynamic flow to the jet pump so as to establish the differential pressure between the zones below and above the modulating plug. A feature is the internal controlled bypass of circulating fluid from the well bore annulus above the modulating plug and alternately from the interior of the drill string for discharge of flushing fluid beneath the modulating plug at and surrounding the drill bit. A feature is the internal and upward bypass of upwardly flowing circulating fluid and chips from the centerflow jet pump and into the well bore annulus above the modulating plug.
An object of this invention is to provide a fluid ejector jet pump that is anti bridging and self cleaning. The jet pump as it is disclosed herein is an annular or peripheral nozzle jet pump, a type to be distinguished from a core type jet pump. Core type jet pumps are those with the nozzle centered within the suction tube ahead of the mixing throat, thereby presenting an obstruction subject to bridging with debris; and heretofore core type jet pumps have become clogged with chips when operated in well drilling tools. Distinctively, annular or peripheral nozzle jet pumps do not present an obstruction to the flow of chip laden fluid, since the suction tube and the mixing throat are of one continuous diamater. As will be seen, the peripheral nozzle opening is annular with absolutely no obstruction to the flow passage of maximized cross sectional area, the suction tube continuing unobstructedly into the mixing throat. In practice, the nozzle entry angle is acute, and as shown the mixing throat is slightly restricted to a smaller diameter than that of the suction tube, so that there is a greatly reduced possibility for the suction tube to become clogged or bridged, since the mixing throat if susceptible to bridging is at the discharge side of the pump subject to nozzle action that will errode and brake up debris.
The modulating plug-stabilizer is free to rotate over the drilling string, although it is not set in the well bore to be non rotating. Accordingly, the modulating plug-stabilizer is embodied in a sleeve journaled to turn on a body member that is part of the tool and located above the drill bit. It is an object therefore to provide for anti friction rotation of the sleeve member on the body member, and to provide fluid communciation to the sleeve member from the interior of the body member so as to controllably expand the modulating plug-stabilizer that isolates the bit-to-bore bottom zone.
The modulating plug-jet pump tool that I provide is a basic element of the oilwell drilling apparatus of the present invention, and its utility as thus far described is associated directly with the bit for increasing the rate of making hole. It is the differential pressure control which is of primary concern and to this end it is an object to selectively apply and remove hydraulic pump pressure so as to control the expansion of the plug-stabilizer into centered flow controlling engagement within the well bore. By controlling the proximate expansion or fit of the plug-stabilizer within the well bore, downward leakage of the well bore annulus fluid under static head is restricted as circumstances require.
In view of the foregoing therefore, it is an object of this invention to provide a plug that is restrictively responsive to the application of static fluid pressure which can be determined by mud pump pressure applied to the aforesaid jet pump. That is, there is a pressure drop that appears as mud pump fluid under pressure passes through the jet pump, with a resultant static pressure applied to the modulating plug at the upstream side of the jet pump to expand the same to a predetermined diameter. In carrying out this invention, I provide an expansible elastomer boot that is reinforced by a cage of longitudinally disposed members which are anchored at opposite ends so as to be stretched when internal fluid pressure is applied. It is the resultant tension applied to the cage of said elongate members that expands the boot radially, a degree according to the amount of static fluid pressure applied. The longitudinal cage members are in the nature of staves in the form of narrow straps of spring steel having a known modulus of elasticity and placed edge to edge to form a barrel or cylinder, when relaxed. When internal fluid pressure is applied to the boot, the cage members stretch under tension and bow outwardly so that controlled radial expansion of the plug occurs restrictively to modulate leakage by the said plug-stabilizer.
It is an object of this invention to separately actuate this basic differential pressure drilling tool, whereby underbalanced drilling conditions beneath the bit can be immediately changed to an over balanced condition, when required. Operation of this basic pressure differential drilling tool at the bit can serve as an immediate "first line of defense" against "blow-outs" and "kicks"; thereby relegating conventional blow-out preventers to a "second line of defense". The basic pressure differential drill tool of the present invention can stop blow-outs and kicks at their source. Still further, the basic modulating plug-stabilizer as it is disclosed herein can inherently operate to slide closely within an open hole and in set casings and the like.